I went to the Andamans in December 1999. I snorkeled while my friends dived, since I hadn't been certified at the time. We were on Havelock Island.
The sea life was spectacular, the beaches white and the waters the range of colours from light green through dark blue. It was more spectacular than Cozumel, Mexico, for instance, with much richer marine life. Not that Cozumel is bad!
Since I was snorkeling, I didn't go very deep -- and can't compare the deeper waters to say Cozumel -- but my diving friend said that the Andamans had some of the best corals he has ever seen. This is someone who has been to the Maldives and Lakshadweep, as to Florida, the Mediterranean and the Red Sea.
Since the Andamans are harder to get to for the average tourist, the infrastructure is more basic but the habitat is more pristine. At least you don't have to deal with cruiseliner loads of tourists.